Fluid pump

ABSTRACT

A fluid pump has a pump body and a displacer, the displacer and the pump body being implemented such that a pump chamber is defined therebetween, the pump chamber having an inlet opening and an outlet opening, neither the inlet opening nor the outlet opening being provided with a check valve. A drive means is provided which positions the displacer periodically at a first and at a second end position. The displacer closes the outlet opening when it occupies its first end position and leaves the outlet opening free when it occupies its second end position and leaves the inlet opening free at both end positions thereof. The displacer, when moving from the first to the second end position, defines a flow-through gap which opens between the displacer and the pump body in the area of the outlet opening in dependence upon the movement, the flow-through gap being defined such that the flow through the outlet opening depends on the pressure in the pump chamber as well as on the respective opening degree of the flow-through gap.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention refers to a fluid pump, i.e. a pump for liquidsand gases.

2. Description of Prior Art

It is known to use positive-displacement pumps for transporting fluids,said positive-displacement pumps consisting of a periodic displacer, apiston or a diaphragm, and two passive check valves. Due to the periodicmovement of the piston or of the diaphragm, liquid is drawn into a pumpchamber through the inlet valve and displaced from said pump chamberthrough the outlet valve. Due to the use of these valves, said knownpumps are complicated and expensive. In addition, the direction oftransport is predetermined by the arrangement of the valves. When thepumping direction of such an arrangement is to be reversed, such knownpumps reuire a change of the operating direction of the valves fromoutside which entails a high expenditure. Such pumps are shown e.g. inJarolav and Monika Ivantysyn; "Hydrostatische Pumpen und Motoren"; VogelBuchverlag, Wurzburg, 1993.

Pumps of this type having a small constructional size and deliveringsmall pumped streams are referred to as micropumps. The displacers ofsuch pumps are typically implemented as a diaphragm, cf. P. Gravesen, J.Branebjerg, O. S. Jensen; Microfluidics--A review; Micro MechanicsEurope Neuchatel, 1993, pages 143-164. The displacers can be driven bydifferent mechanisms. Piezoelectric drive mechanisms are shown in H. T.G. Van Lintel, F. C. M. Van de Pol. S. Bouwstra, A PiezoelectricMicropump Based on Micromachining of Silicon, Sensors & Actuators, 15,pages 153-167, 1988, S. Shoji, S. Nakagawa and M. Esashi, Micropump andsample injector for integrated chemical analyzing systems; Sensors andActuators, A21-A23 (1990), pages 189-192, E. Stemme, G. Stemme; Avalveless diffuser/nozzle based fluid pump; Sensors & Actuators A, 39(1993) 159-167, and T. Gerlach, H. Wurmus; Working principle andperformance of the dynamic micropump; Proc. MEMS'95; (1995), pages221-226; Amsterdam, The Netherlands. Thermopneumatic mechanisms fordriving the displacers are shown in F. C. M. Van de Pol, H. T. G. VanLintel, M. Elwenspoek and J. H. J. Fluitman, A Termo-pneumatic MicropumpBased on Micro-engineering Techniques, Sensors & Actuators, A21-A23,pages 198-202, 1990, B. Bustgens, W. Bacher, W. Menz, W. K. Schomburg;Micropump manufactured by thermoplastic molding; Proc. MEMS'94; (1994),pages 18-21. An electrostatic mechanism is shown in R. Zengerle, W.Geiger, M. Richter, J. Ulrich, S. Kluge, A. Richter; Application ofMicro Diaphragm Pumps in Microfluid Systems; Proc. Actuator '94;15.-17.6.1994; Bremen, Germany; pages 25-29. Furthermore, the displacerscan be driven thermomechanically or magnetically.

As is also shown in the above-mentioned publications, either passivecheck valves or special flow nozzles can be used as valves, said checkvalves and said flow nozzles being both expensive and complicated. Thedirection of transport of micropumps can be reversed without forciblycontrolling the valves, simply by effecting control at a frequency abovethe resonant frequency of said valves. In this context R. Zengerle, S.Kluge, M. Richter, A. Richter; A Bidirectional Silicon Micropump; Proc.MEMS '95; Amsterdam, Netherlands; pages 19-24, J. Ulrich, H. Fuller, R.Zengerle; Static and dynamic flow simulation through a KOH-etched microvalve; Proc. TRANSDUCERS '95, Stockholm, Sweden, (1995), pages 17-20,should be taken into account. The cause of this effect is a phasedisplacement between the movement of the displacer and the opening stateof the valves. If the phase difference exceeds 90°, the opening state ofthe valves is anticyclic to their state in the normal forward mode andthe pumping direction is reversed. A change of the operating directionof the valves from outside of the type required when macroscopic pumpsare used can be dispensed with. The decisive phase difference betweenthe displacer and the valves depends on the drive frequency of the pumpon the one hand and on the resonant frequency of the movable valvemember in the liquid surroundings on the other.

One disadvantage of this embodiment is to be seen in the fact that, uponconstructing the valves, a compromise has to be found between themechanical resonance in the liquid surroundings, the flow resistance,the fluidic capacity, i.e. the elastic volume deformation, theconstructional size and the mechanical stability of these valves. Itfollows that these parameters, each of which may influence the pumpingdynamics, cannot be ajusted to an optimum value independently of oneanother and part of them is opposed to a desired further miniaturizationof the pump dimensions.

A general disadvantage entailed by the use of pumps with passive checkvalves is also the fact that, when switched off, the pumps do not blockthe medium to be transported. If the input pressure exceeds the outputpressure by the pretension of the valves, the medium to be pumped willflow through the pump.

Micropumps using special flow nozzles have the disadvantage that theyhave a very low maximum pumping efficiency in the range of 10 to 20%.

DE-C 19534378.6 discloses a fluid pump comprising a pump body, adisplacer and an elastic buffer. The displacer closes an inlet arrangedin said pump body when occupying a first end position and leaves saidinlet arranged in the pump body free when occupying a second endposition. The known pump permits a net flow through an outlet which isalso provided in the pump body. The buffer means bordering on the pumpchamber formed by the displacer and the pump body makes the known fluidpump expensive and complicated.

Esashi, Shoji and Nakano describe in the article "Normally closedmicrovalve and micropump fabricated on a silicon wafer", Sensors andActuators 20 (1989), pages 163-169, a gas microvalve which is normallyclosed. The valve consists of a glass plate having arranged therein agas outlet opening which is adapted to be closed by means of asilicon-mesa structure that is provided with a valve seat and that isadapted to be operated by a piezoelectric drive. The silicon layer, inwhich the silicon-mesa structure is formed, and the glass plateadditionally define a continuous channel between the gas outlet openingand a gas inlet opening formed in the silicon layer. The above-mentionedpublication also describes a diaphragm-type micropump consisting of twoone-way valves and a diaphragm with a piezoelectric drive means.

SUMMARY OF THE INVENTION

Starting from the prior art cited, it is the object of the presentinvention to provide an efficient fluid pump having a simple structuraldesign and to provide a method for operating such a pump.

In accordance with a first aspect of the present invention, this objectis achieved by a fluid pump, comprising:

a pump body;

a displacer, said displacer and said pump body being implemented suchthat a pump chamber is defined therebetween, said pump chamber having aninlet opening and an outlet opening, neither said inlet opening nor saidoutlet opening being provided with a check valve;

a drive means positioning the displacer periodically at a first and at asecond end position,

the displacer closing said outlet opening when it occupies its first endposition and leaving said outlet opening free when it occupies itssecond end position and leaving the inlet opening free at both endpositions thereof,

said displacer, when moving from the first to the second end position,defining a flow- through gap which opens between the displacer and thepump body in the area of the outlet opening in dependence upon saidmovement, said flow-through gap being defined such that the flow throughthe outlet opening depends on the pressure in the pump chamber as wellas on the respective opening degree of said flow-through gap.

In accordance with a second aspect of the present invention, this objectis achieved by a method of operating a fluid pump having theconstruction mentioned above, wherein

during a suction phase in the course of which the displacer is movedfrom the first to the second end position an essentially linearlyincreasing voltage is applied to the drive means, and

at the beginning of a pressure phase in the course of which thedisplacer is moved from the second to the first end position the voltageapplied to the drive means is abruptly switched off.

The present invention provides a fluid pump comprising a pump body and adisplacer, which is adapted to be periodically positioned at a first andat a second end position with the aid of a drive means, said displacerand said pump body being implemented such that a pump chamber is definedtherebetween, said pump chamber having an inlet opening and an outletopening. The displacer closes the outlet opening when it occupies itsfirst end position and leaves said outlet opening free when it occupiesits second end position. When the displacer moves from the first to thesecond end position, it opens a flow-through gap between the pump bodyand the displacer in the area of the outlet opening. The pump body ispreferably implemented in the form of a plate including said inlet andoutlet openings, whereas the displacer is provided with a recessdefining the pump chamber.

The pumping efficiency can be optimized by adapting the cross-sectionalareas of the inlet and outlet openings and by controlling the timing ofthe driving of the displacer into the first and second end positions.The displacer can be driven by a piezoelectric bending converter or apiezoplate secured in position by means of an adhesive or it can also bedriven electrostatically.

A fluid pump according to the present invention has a simple structuraldesign which may consist of a single structured silicon chip. Thispermits a reduction of the costs for processing the silicon componentsand also a reduction of mounting costs. A further saving of costs isachieved when the pump according to the present invention is producedfrom plastic material by precise mechanical processes, such as injectionmoulding, etc.

The displacer of the fluid pump according to the present invention isdriven by a driving voltage having a polarity of such a nature that thedisplacer is raised. When the pump has been switched off, the polarityof the driving voltage can be reversed, whereby the outlet opening isclosed with a defined, high contact force. Hence, the outlet openingdefines together with the displacer an active valve which represents anessential advantage in comparison with passive valves. By introducing asmall buffer volume into the pump chamber, it is further possible toreverse the pumping direction of a fluid pump according to the presentinvention, whereby the use of a second pump can be dispensed with inmost cases.

BRIEF DESCRIPTION OF THE DRAWING

In the following, preferred embodiments of the present invention will beexplained in detail making reference to the drawings enclosed, in which

FIG. 1 shows a cross-sectional view of an embodiment of a fluid pumpaccording to the present invention;

FIG. 2 shows the pressure in the pump chamber of a fluid pump accordingto the present invention during a suction phase and a pressure phase;

FIG. 3 shows a graph showing the dependence of the flow through theoutlet opening on the gap width;

FIGS. 4a to 4d show representations of the transient processes takingplace in the fluid pump of FIG. 1;

FIG. 5 shows the dependence of the flow through the inlet and outletopenings in the case of various pressure differences;

FIG. 6a to 6c show different control voltages for driving the displacerof a fluid pump according to the present invention;

FIG. 7 shows a graph showing a special pressure characteristic in thepump chamber of a pump according to the present invention;

FIGS. 8 and 9 show various embodiments of a fluid pump according to thepresent invention;

FIGS. 10a to 10d show four further embodiments used for controlling thedisplacer according to the present invention;

FIGS. 11a to 11d show representations of the transient processes takingplace in a fluid pump according to the present invention including asmall buffer volume in the pump chamber; and

FIG. 12 shows a cross-sectional view of a further embodiment of a fluidpump according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a preferred embodiment of a fluid pump according to thepresent invention. The pump comprises a pump body 10 and a displacer 12.The pump body has formed therein an outlet opening 14 having a width wand an inlet opening 16. The outlet opening 14 and the inlet opening 16can have an arbitrary shape, e.g. a square, a round, a rectangular or anellipsoid shape. The displacer 12 is secured to the pump body 10 and isprovided with a recess defining together with said pump body 10 a pumpchamber 18. The pump body 10 and the displacer 12 can have e.g. acircular shape.

The displacer 12 is adapted to be moved to and fro into first and secondend positions by means of a piezo bending converter 20 consisting ofpieoelectric ceramics. The piezo bending converter 20 is secured to thedisplacer 12 e.g. by means of an adhesive 22. The displacer 12 definesat its central, thicker portion 13a valve together with the outletopening 14, said outlet opening 14 being closed at the first endposition of the displacer 12 and open at the second end position of thedisplacer 12.

When a voltage is applied to the piezo bending converter 20, thedisplacer 12 will move upwards to the second end position and open theoutlet opening 14. When the voltage is switched off, the displacer 12will move downwards to the first end position where it closes the outletopening 14. The inlet opening, which can be implemented as an orifice,is permanently open.

A general consideration of the mode of operation of the pump accordingto FIG. 1 follows. As the displacer 12 moves, both a pressure p in thepump chamber 18 and a gap height h at the outlet opening 14 change. Theflow through the outlet opening depends on these two factors, thepressure p and the gap height h. A simplified consideration results in aflow rate φ proportional to ph³, the relationship in the case of a moregeneral consideration being p^(x) h^(y) where x and y are arbitrarynumbers.

When the temporal integration over the flow is different for the openingand closing processes of the displacer 12, a net fluid transport in anindicated pumping direction through the outlet opening 14 will resultwhen the displacer 12 is actuated periodically. This net fluid transportcan be calculated by a mathematical integration over the flow rate.

FIG. 2 shows the pressure characteristic with time in the pump chamber18 when the piezo bending converter 20 is controlled by a square-wavevoltage. When the voltage is applied, a underpessure is first created inthe pump chamber 18, said underpressure decreasing as the degree ofdisplacement of the displacer 12 increases. The displacement of thedisplacer 12 corresponds to the gap height h. When the voltage isswitched off, or, alternatively, reversed, an excess pressure isobtained in the pump chamber 18, said excess pressure decreasing whenthe displacement of the displacer 12 decreases.

The time-dependent flows through the two openings in the pump body 10,the outlet opening 14 and the inlet opening 16, are now fundamentallydifferent. Whereas the flow through the inlet opening 16 is onlydetermined by the pressure characteristic in the pump chamber 18, theflow through the outlet opening 14 depends on the instantaneous pressurep in the pump chamber as well as on the instantaneous gap height h atthe outlet opening 14.

The amount of flow through the inlet opening or inlet orifice is givenby ##EQU1## in a first approximation, where A_(orifice) is thecross-sectional area of the inlet opening or orifice 16, μ is ageometry-dependent, dimensionless outflow coefficient, ρ is the densityof the fluid, p₁ is the pressure in the inlet ending in the inletopening (cf. FIG. 1), and p is the pump chamber pressure.

The flow through the outlet opening can, however, approximately beconsidered to be a laminar gap flow, which is given by: ##EQU2## Where wis the width of the outlet opening, h is the displacement of thedisplacer, b is the length of the respective gap (cf. FIG. 1), η is theviscosity of the fluid and p₂ is the pressure in the outlet ending inthe outlet opening (cf. FIG. 1).

The flow through the outlet opening in dependence upon the gap height his shown for a constant pressure difference in FIG. 3. Especially forlow gap heights h, the flow rate is drastically reduced.

The fact that the flow through the outlet opening depends on the twoindependent variables, viz. the pump chamber pressure p and the gapheight h, is of decisive importance for the pumping mechanism of thefluid pump according to the present invention.

In FIGS. 4a to 4d, the transient processes occurring during the suctionphase and during the pressure phase in the pump according to FIG. 1 areshown in the form of a diagram.

FIG. 4a shows the curve of the displacer movement; FIG. 4b shows thecurve of the pump chamber pressure p; FIG. 4c shows the flow through theinlet opening and FIG. 4d the flow through the outlet opening.

Suction Phase

When the voltage applied to the piezo bending converter is switched on,an underpressure will immediately prevail in the pump chamber withoutthere being any appreciable upward movement of the displacer. This isshown at the time 0.0 in FIGS. 4a and 4b. Since the outlet opening isstill closed at this time, no fluid will flow through said opening. Thefluid will first flow exclusively through the inlet opening into thepump chamber (cf. time 0.0 in FIGS. 4c and 4d). Only an increasingmovement of the displacer and a resultant increase in the gap height hwill cause an additional flow of fluid through the opening that is beingformed. Since, however, the underpessure in the pump chamber decreasesagain during the movement of the displacer, the fluid volume flowingthrough the outlet opening is comparatively small because the flow isproportional to the product ph³.

Pressure Phase

When the voltage applied to the piezo bending converter is switched off(time 2.0 in FIGS. 4a to 4d), an excess pressure will immediatelyprevail in the pump chamber without any appreciable downward movement ofthe displacer taking place. In this condition, the outlet opening isopen and a comparatively high excess pressure prevails simultaneously inthe pump chamber. Hence, the product ph³ is comparatively large. Itfollows that the amount of fluid flowing through the outlet opening outof the pump chamber in the pressure phase exceeds by far the amount offluid which has flown through the outlet opening into the pump chamberin the suction phase, as can be seen from FIG. 4d. This figure clearlyshows the dissymmetry of the flow through the outlet opening in thepressure phase and in the suction phase and the resultant net flowthrough the outlet opening.

The net pumping effect of the fluid pump according to the presentinvention is based on the circumstance that different amounts of fluidflow through the gap between the displacer and the outlet opening whilethe outlet opening is being opened, i.e. during the suction phase, andwhile the outlet opening is being closed, i.e. during the pressurephase. The reason for this is that the flow through the outlet openingdepends both on the pressure in the pump chamber and on the gap height hbetween the displacer and the pump body.

In the following, alternative embodiments of the present invention willbe described.

The pumping efficiency of a pump according to the present invention,i.e. the amount of fluid pumped per pumping cycle, and the maximumcounterpressure that can be achieved in the pump chamber can be variedby modifying the cross-sections of the two openings. Especially areduction of the cross-sectional area of the inlet opening relative tothe cross-sectional area, i.e. the width w, of the outlet opening willresult in an increase of the maximum pressure. The pressure efficiencycan additionally be improved by an optimized characteristic of thecontrol voltage.

This consideration is based on the observation that the flowcharacteristic of the inlet opening, which is proportional to √p, has analmost perpendicular gradient starting from its origin, whereas, in thecase of a constant gap height h, the flow through the outlet openingincreases only linearly as the pressure increases. These effects areshown in FIG. 5. The flow through the inlet opening will thereforealways predominate when the pressure differences are small. It followsthat, when the pressure in the pump chamber is deliberately kept lowduring the suction phase and deliberately kept high during the pressurephase, it will be possible to enhance the pumping efficiency.

In the case of a given control voltage U, the pressure in the pumpchamber adjusts itself in such a way that there is an equilibrium offorces beween the pump drive, the intrinsic strain of the displacer andthe hydrostatic pressure of the fluid in the pump chamber. FIGS. 6a, 6band 6c, show two possibilities of advantageously modifying the pressurein the pump chamber by a suitable control voltage.

A feature which the voltage characteristics shown in FIGS. 6a to 6c havein common is a linear voltage increase during the suction phase andabrupt switching off of the voltage during the pressure phase. In thecase of the voltage characteristic of FIG. 6c, the polarity of thevoltage is also deliberately reversed at the beginning of the pressurephase, whereby the pressure in the pump chamber will be increased beyondnormal. By means of such control voltages, the pumping efficiency can beincreased deliberately. In addition, it is also clearly evident that thedisplacer can be closed either by its mechanical restoring force alone,due to its deformation (passively), or via the drive means (actively).

Hence, the decisive point of the pumping mechanism according to thepresent invention is to be seen in the fact that, as the displacermoves, both the pressure p in the pump chamber and the height of theflow gap at the outlet opening change. The flow through the outletopening is composed of these two factors. A simplified considerationresults in a flow rate φ proportional to ph³ ; in the case of a moregeneral consideration, the flow rate is proportional to p^(x) h^(y)where x and y are arbitrary numbers.

It is explicitly pointed out that all relationships p^(x) h^(y) betweenthe pump chamber pressure p and the gap height h result in a pumpingeffect, provided that different values for the amount of fluid flowingthrough the outlet opening are obtained during the integration in thecourse of the opening and closing processes of the outlet opening by thedisplacer. Hence, it is also evident that a laminar gap flow through thevalve is not a prerequisite for the pumping function. A pumping effectis also possible when the flow in question is a turbulent flow or anymixed kind of flow.

In order to achieve a good pumping efficiency, special pressurecharacteristics in the pump chamber may be advantageous. A pressurecharacteristic of this type is shown in FIG. 7. Such a pressurecharacteristic can be achieved e.g. by means of an electrostatic driveor by means of a deliberate modification of the control voltage (cf.FIG. 6).

FIG. 8 shows an alternative embodiment of the present invention. Thepump body 100 of this embodiment consists of a fluidic base plate withintegrated channels 105 and 107, which end in an outlet opening 140 andan inlet opening 160, respectively. A structured silicon chip serves asdisplacer 120, said silicon chip being secured to the fluidic base plateand being implemented such that it closes the outlet opening 140 at afirst end position and leaves said outlet opening free at a second endposition. In addition, a pump chamber 180 is defined by a recessprovided in the displacer 120. The component used as a drive means inthe embodiment shown in FIG. 8 is a piezoelectric ceramic plate, whichis secured to the displacer and which can be provided with a layer forselective bonding on the upper surface thereof.

In FIG. 9 a further embodiment of the present invention is shown, whichcorresponds to the embodiment of FIG. 8 with the exception of the drivemeans used for the displacer. In the embodiment shown in FIG. 9, anelectrostatic drive of the displacer has been realized. For thispurpose, a counterelectrode 210 is arranged in spaced relationship withthe displacer 120 above the side of said displacer located opposite thepump body 100, said counterelectrode being used for moving the displacerto the first and to the second end position. An electrostatic drive hasthe advantage that it permits, simply on the basis of the non-linearelectrostatic driving forces, a highly unsymmetrical pressurecharacteristic in the pump chamber during the suction phase and duringthe pressure phase, said pressure characteristic being shown e.g. inFIG. 7.

In FIGS. 10a to 10d further embodiments used for controlling thedisplacer are shown. As far as these embodiments are concerned, it canbe differentiated between a pointwise or an areawise introduction offorce. Another differentiating criterion in connection with such controlmeans is whether they permit a forcible control or a control allowing areaction. When a forcibly controlled displacer is used, there will be noreaction coupling between the displacer position and the pump chamberpressure.

FIG. 10a shows a drive means for a pointwise introduction of forcewithout a forcibly controlled displacer. FIG. 10b shows a drive meansfor an areawise introduction of force without a forcibly controlleddisplacer. In FIGS. 10c and 10d, respectively, drive means are shown fora pointwise and an areawise introduction of force with a forciblycontrolled displacer.

In order to increase the pumping efficiency, it may also be advantageousto implement the orifice, i.e. the inlet opening, as a flow nozzle, suchflow nozzles being normally provided in so-called diffusor nozzle pumps.This will have an additional positive effect on the pumping direction.

If elastic components are arranged within the pump chamber or outside ofsaid pump chamber, this will influence the pressure characteristic inthe pump chamber as well as the flow rates through the inlet and outletopenings. The elastic components can e.g. be an elastic diaphragm or anelastic media entrapment, such as gas. The transient processes takingplace in a pump in this case are shown in FIG. 11.

When the operating frequencies are high, the region of theeigenfrequency of these elastic components in their fluid surroundingswill be reached. This will result in a phase displacement between thepressure characteristic in the pump chamber and the movement of thedisplacer. The relative amounts of the forward and reverse flow throughthe outlet opening change and the pumping direction is reversed.

The fluid to be moved in the fluid lines is one of the factorsdetermining the resonant frequency. This has the effect that e.g. thethreshold frequency, from which a reversal of the direction of transportoccurs, becomes lower because of the larger fluid mass as the length ofthe fluid lines increases. By deliberately introducing elasticcomponents in the area outside of the pump chamber, this undesiredcoupling between the resonant frequency and the fluid lines can besuppressed.

When only small elastic buffer volumes are present in the pump chamber,the pumping mechanism described will be disturbed very little by saidbuffer volumes, as can be seen in FIGS. 11a to 11e. The buffer volumemust not exceed a specific size, since, otherwise, the pumping mechanismaccording to the present invention would no longer be guaranteed.

When, in a fluid pump according to the present invention, no bufferelement is provided in or on the pump chamber, the dynamic behaviour ofthe moving fluid column can be used for the purpose of reversing thepumping direction. When the pump is operated at a frequency whichcorresponds to the resonant frequency of the moving fluid column, thiswill result in a phase displacement between the pressure and themovement of the fluid, said phase displacement causing a reversal of thedirection of flow.

A reversal of the pumping direction can also be achieved by making useof the dynamic behaviour of the displacer. When the pump is operated ata frequency which corresponds to the resonant frequency of thedisplacer, a phase displacement between the force driving the displacerand the movement of the displacer will cause a reversal of the pumpingdirection.

FIG. 12 shows a further embodiment of a fluid pump according to thepresent invention. In the fluid pump shown in FIG. 12, a pump chamber380 is formed as a capillary gap between a pump body 310 and a displacer320. Filling can substantially be simplified on the basis of such anarrangement, since a fluid is drawn into the pump chamber due to thecapillary forces. In FIG. 12, the drive mechanism for the displacermeans is not shown.

A fluid pump according to the present invention can also be providedwith a pressure sensor through which the fluid pump is maintained in theideal operating range. The pessure sensor can be arranged in or on thepump chamber so as to pick up the pressure prevailing in said pumpchamber. For this purpose, the pressure sensor can e.g. be integrated inthe displacer 320, which is implemented as a diaphragm, in theembodiment shown in FIG. 12. The drive means of the micropump can thenbe brought into the respective optimum operating range via a controlcircuit.

What is claimed is:
 1. A fluid pump comprising: a substantially flat,plate-shaped pump body having an outlet opening and an inlet openingformed therethrough in parallel and adjacent to each other, wherein theoutlet opening is arranged on center of said pump body, neither saidinlet opening nor said outlet opening being provided with a check valve;a plate-shaped displacer connected circumferentially to said pump bodysuch that a pump chamber is defined therebetween, said pump chamberbeing substantially symmetrical with respect to said outlet opening;drive means for positioning the displacer periodically at a first and ata second position, said drive means having a motive element, said motiveelement having first and second ends respectively, the first end beingattached to the center of the displacer, the second end being attachedto a wall of the pump body; the displacer closing said outlet openingwhen it occupies its first end position and leaving said outlet openingfree when it occupies its second end position and leaving the inletopening free at both end positions thereof, said displacer, when movingfrom the first to the second end position, defining a flow-through gapwhich opens between the displacer and the pump body in the area of theoutlet opening in dependence upon said movement, said flow-through gapbeing defined such that the flow through the outlet opening depends onthe pressure in the pump chamber as well as on the respective openingdegree of said flow-through gap.
 2. A fluid pump according to claim 1,wherein the pump body is implemented in the form of a plate includingsaid inlet and outlet openings, and that the displacer is provided witha recess defining together with the pump body the pump chamber.
 3. Afluid pump according to claim 1, wherein the pump body is implemented inthe form of a plate having inlet and outlet openings, said pump bodybeing additionally provided with a recess defining together with thedisplacer the pump chamber.
 4. A fluid pump according to claim 1,wherein the pump chamber is implemented as a capillary gap.
 5. A fluidpump according to claim 1, wherein the cross-sectional area of the inletopening is reduced in comparison with the cross-sectional area of theoutlet opening.
 6. A fluid pump according to claim 1, wherein the drivemeans is a piezoelectric bending converter, said piezoelectric bendingconverter bending upwards when voltage is applied, said central portionof said displacer moving upwards and opening said outlet opening, saidconverter returning to said first end position when voltage is switchedoff and closing said outlet opening.
 7. A fluid pump according to claim1, wherein the drive means consists of a piezo plate applied to the sideof the displacer located opposite the pump body.
 8. A fluid pumpaccording to claim 1, wherein the drive means is an electrostatic drive.9. A fluid pump according to claim 1, wherein the displacer closes theoutlet opening passively when the pump has been switched off.
 10. Afluid pump according to claim 1, wherein the displacer closes the outletopening by applying a voltage with opposite sign to the drive means. 11.A fluid pump according to claim 1, wherein a pressure sensor is arrangedin or on the pump chamber, said pressure sensor being used for forming acontrol circuit.